Implanted medical devices, such as pacemakers and implantable cardioverter defibrillators (“ICDs”), produce life-saving therapeutic outputs for the heart. Such devices, either preferably or, in the case of an ICD, necessarily, sense physiologic conditions of the body in order to adjust their operation for the benefit of the patient. Both such devices sense electrical activity native in the body in the atria and the ventricles. Such devices may also sense additional body information such as that related to activity level or respiration. Again, these devices may adjust their output in order to provide an improved benefit for the patient.
Detecting such physiologic conditions externally to the body is also desirable, and such needs could be separate and apart from the operation of the implantable medical device. Sensing the underlying physiologic condition could detect arrhythmias, hemodynamics and other functions. Further, detecting the underlying physiologic condition could also be useful for use in conjunction with the device, as, for example, detecting device malfunction.
While implanted medical devices, such as pacemakers and ICDs, are often programmed to operate automatically in response to their sensed physiologic conditions, it is difficult to monitor these devices and determine, from a location external to the patient, such underlying physiologic conditions. For example, naturally occurring electrical signals in the atria and ventricles are normally have an amplitude in the range of several millivolts. However, such signals detected at the skin surface are ordinarily in the range of millivolts to microvolts. While the implanted medical device can easily detect such native signals in the millivolt range, it is more difficult and cumbersome to detect the same signals at the skin surface.
Prior art implanted medical devices have communicated information about their operation external of the body by utilizing radio telemetry, typically in the 175 kiloHertz range. Such technique is widely used to externally interrogate the memory contained in such an implanted medical device. This technique requires a relatively sophisticated instrument such as a programmer which is capable of interacting with the implanted medical device along with computational capabilities in the instrument to decode the data in the telemetry stream.
Another prior art technique used to communicate information about the implanted medical device externally from the body is a “patient alert” type feature. In this technique, the implanted medical device may emit an audible tone intended to alert the patient if certain physiologic or medical device conditions are sensed, such as a lead problem or a low battery. However, this technique requires that the patient listen for generated audible tone, determine what the audible means and, then, contact a health care worker.